Multi-stage igniter charge



MULTI-STAGE IGNITER CHARGE Filed June 29, 1959 ein 3e 3B x-Go 54 gis 5a@27 5| Gl )52 Bc/ f.- INVENTOQ. i2-Z- QOBEQT K. ASDLUND BY @MJMMATTORNEYS Filed June 29, 1959, Ser. No. 823,586

1 Claim. (Cl. 102-70) This invention relates generally to igniters ofthe type having an igniter charge within a housing of sufficientstrength to resist rupture during ignition combustion, and also havingone or more nozzles `for discharging igniting gases from said housing;such igniters are generally used for igniting bodies of solid propellantsupported within rocket housings.

More particularly, this invention is concerned with an igniter of thetype described in which the duration and pressure of the jets ofigniting gas are controlled, at least .in part, by an ignition chargecomprised of an initial stage of rapid burning igniter material enclosedin or surrounded by a main ignition charge made up at least in majorpart of ignition pellets enveloped in slow burning or combustionresistant material.

y In the preferred species of the invention, the main igniter charge isannular in shape and surrounds a core of initial igniter charge, intowhich a squib means discharges, with or without the aid of confining ordirecting barrier lwalls.

In rockets or missiles in which solid propellant is used, the propellantis ordinarily supported in a chamber inside a housing or case with freespace around the propellant material. The means of support may be aspider or the like, but the present invention is not concerned with thenature of the propellant or of its support, or general structure, butonly with the need for igniting it by means of hot ignting gases.

It is important for suitable ignition that the igniting gases beprojected at or on the solid propellant in a desired manner for asuitable igniting time of a few milliseconds. It is desirable forreliable ignition, without failure and without irregular and improperburning of the propellant, that the igniting gases are supplied in avery roughly constant fashion for the brief but perceptible ignitingperiod. Of course, the pressure of the gases will change substantiallyduring the igniting period, but it is desirable to avoid very high peakpressures which then trail off to much lower pressures early in theignition period.

One excellent solution for the .problem has been to provide a strongigniter housing for the igniter charge,

with a plurality of nozzle openings directed toward the propellant, andadapted to project a number of jets of hot the exposed surfaces of thesolid propellant. Such an igniter housing may be in the form of anignition charge container mounted in the end of the rocket housing,andprovided with an electrically controlled squib ignitng means in itsouter end, external to said rocket housing.

The present invention uses such a housing but only as one coacting partof a combination, the principal novel parts of which are the combustibleigniter materials themselves, which are shaped into charges providingsustained and controlled ignition burning.

In the past, the igniter housing has had to be made heavy enough towithstand high peak pressures, or has been constructed of lightmaterials which would blow out upon ignition, with resultant inferiorquality of ignition.

Control of pressure and duration of ignition llame has been attempted bymaking use of heavier and more comf'plex igniter housings, with morecomplex and expensive nozzle arrangements. Such complexity naturally1ntates Patent ice t creases expense and sometimes introduces sources offailure.

A practical yform for the igniter charge material is in pellets ratherthan in powder. However, some failures in the past have been traced ltoattrition of said pellets within the igniter housing, during transportand storage prior to use. Although some of this attrition can be avoidedby the introduction of cushioning materials, cushioning alone has provenineffective to avoid substantial attrition, and failures resultingtherefrom.

Other proposals have involved the use of slow burning powders, delayedignition structures, various types of fuses, etc., but all have provenrather complicated and expensive for the purpose intended.

It is a major object of the present invention to provide an igniterproducing propellant-igniting gases of controlled duration and limitedpressure, by means of a novel structure of the combustible ignitermaterial itself.

It is another object of the invention to provide an igniter chargestructure which achieves the results of two or more stages ofcombustion, with extremely simple and .economical arrangements of thecharge materials, and the igniter in which the ignition charge orcharges are largely protected from attrition, without interfering withtheir quick and reliable ignition by standard squib ignition means.

The foregoing and other. objects are accomplished with standard ignitermaterials. Any of the solid igniter materials ordinarily used ingranular or pelletized form may be employed in the practice of theinvention. Such igniter materials are generally much more readilyignited than the propellant employed as rocket fuel, which is to beignited by the igniter. Also, such igniter materials ordinarily have aburning rate which is much higher than that of propellant materials orcertain other solid oxidant compositions which can be used inassociation with the rapidly burning igniter materials.

In the present invention, at least two stages of igniter `combustion areapproximated by pellets of the same rapidly burning igniter material,some of said pellets being uncoated and nnshielded from the squibignition, and others being enveloped by a coating or a solid matrix bedof relatively slow .burning material, which may be a slower burningigniter material, propellant material, or

in a somewhat more elaborate form, approximating threestage ignitionburning.

In FIGURE l, an igniter indicated generally by the Ynumeral 10 isenclosed in a two-piece igniter housing 11,

which is generally cylindrical in shape, and is comprised of a lower cup12 on to which is threaded an inverted cup shaped cover 13.

The lower cup container 12 may be threadably received into the end of arocket housing, in the same manner as is illustrated in FIGURE 2 for cup32 which is received in a rocket housing 60.

3 ing, as is shown for O-ring 61 placed in recess 62, in connection withigniter cup 32 in FIGURE 2.

The cover 13, which may be referred to as the inner end of the igniterhousing 11, since it projects into the interior of the rocket housingwith which it is used, is provided with a number of openings 15, whichserve as nozzles, and may project jets of igniting gases in a number ofdirections, as desired, when the igniter has been ignited. The outer endof the cup 12, which is normally exposed outside the rocket housing, isprovided with any suitable squib means 16. The commonest type of squibwill be comprised principally of a readily combustible powder packedaround an electrical heating element which is connected by means ofleads to a suitable source of igniting power and control switch.However, the invention is not limited as to squib means, and any fusesquib device or the like which meets the engineering requirements of aparticular case may be employed.

The interior 17 of the igniter housing 11 is filled with an assortmentof three kinds of small pellets forming the igniter charge. Thesepellets may be identified las rapid burning pellets 18, which arepreferably relatively small, and are uncoated and unprotected from thefire of squib 16. These pellets serve to insure initial fixing of theigniter charge, and a few of them may be scattered through the entirecharge, if desired, to insure continued burning. Their only essentiallocation, however, is in the immediate vicinity of the squib 16. Somemust be concentrated in that region in order to assure initial firing,even if there are no rapidly burning pellets 18 in the remainder of theinterior 17.

Slow burning pellets, which may also be referred to as sustainingpellets, are preferably, `although not necessarily, substantially largerthan the rapid burning pellets. Their essential difference is that theyare covered over all or a large part of their exterior surface with athin coating of a silicone paint or the like. In the embodimentillustrated, the slow burning pellets illustrated at 19, have beenentirely covered with a spray coating of silicone paint, although theirinterior composition is identical to that of the rapid burning pellets18. It is an essential feature of the invention that the slow burningcoated pellets 19 be distributed around or enclosing the centralconcentration 20 of rapid burning pellets 18.

Typical pellet size for most igniters is about Ms inch in diameter by ainch long for the smaller pellets, up to 1A inch in diameter and 1A inchlong for the larger pellets, particularly those which are coated and areto serve as the sustained burning stage of the igniter. The pellets mayrange in weight from only a few grams up to over one hundred grams. Theyshould be sufiiciently numerous so as to provide a readily distributablestructure, usually fteen to two hundred pellets, and over fifty percentof the material should be concentrated in the coated sustaining pellets.

Coating of the pellets 19 has been found to have a beneficial effect onthe prevention of attrition, in addition to the suppression ofcombustion rate which produces the ignition of desirable duration andpressure limitation. However, in the particular embodiment describedherein in connection with FIGURE l, close packing of the pellets 18 and19 are achieved without much attrition resulting by random scattering ofmany chunks of cushioning material 21, which may be referred to ascushion pellets, although it is not compressed or pelletized in anysense, but it is very soft.

The entire igniter charge, which may be referred to collectively by thenumeral 22, may appear at first to be packed as au entirely randomassortment of rapid burning pellets 18, slow burning coated pellets 19,and cushioning material 21. However, as previously pointed out, it isactually packed as a more or less annular pack comprised predominantlyor exclusively of slow burning uncoated pellets y19, so far as ignitermaterial is concerned, enclosing a core 20 of rapid burning pellets 18disposed immediately adjacent to the igniter 16.

The entire charge 22 is enclosed in a moisture-impervious film, whichprotects it from the moisture or other airborne impurities which mightotherwise enter through the nozzle openings 15 during storage prior touse. The film is indicated generally by the numeral 23, and may beunderstood to be any of a variety of suitable materials. For example,the film may be aluminum foil, a sheet of plastic such as Mylar plastic,or even paper impregnated with some suitable moisture resistantimpregnating material. Also, the film 23 may be a painted coatingapplied to the igniter charge `after its formation, and before or afterinsertion into the igniter housing 11. It should also be understood,that the film 23 may be actually somewhat thicker than an ordinary filmand may actually be comprised not only of a waterproofing material but acushioning lining of such materials as felt, plastic foam, or the like.

The operation of the igniter of FIGURE 1 will be obvious to thoseskilled in the art of igniters. The igniter housing 11 is constructed ofsufiicient strength to resist rupture at the igniter combustionpressures. Consequently, when the squib 16 is fired, the central core 20which is comprised of a concentration of the relatively small anduncoated rapid burning pellets 18, is ignited as the first stage ofignition.

The pressure of the hot gases produced breaks through the light film. 23and jets from the nozzle openings 15 to begin ignition of propellant.Within a short time, the increased pressure and temperature within theigniter housing 11 advances combustion to the second ignition charge 22,comprised of the coated and relatively slower burning pellets 19distributed in an annulus around the central core 20.

FIGURE 2 illustrates a three-stage embodiment of the igniter of theinvention. The three-stage igniter indicated generally by the numeral 30has its charge enclosed in a housing indicated generally by the numeral31, comprised of a cup 32 and a cap 33, cup 32 being threadably receivedin the end of the housing 60, seating against the O-ring 61, in theannular recess 62.

Cup 33 is provided with a plurality of nozzle openings 35 by means ofwhich the igniter 30 projects jets of igniting gases against thepropellant charges 64 and 63 during ignition. Propellant charges 64 and63 are supported within the rocket housing 60 with a substantial regionof clear space between them and the igniter 30, and between each other.Such support may be made within the housing 60 by means of supportspiders, housing structure or the like, all in the manner well known tothose familiar with the construction of rockets, and not illustratedherein since they form no part of the present invention. It should onlybe noted that the spacing illustrated in FIGURE 2 is not proportionateto actual dimensions; the spacing has actually been reduced very much inorder to make illustration convenient.

An electrical squib means 36 is threadably received in a threaded bore36a in the center of the outer end of the igniter housing cup 32.

The igniter means 36 is used to ignite an igniter charge identifiedgenerally by the numeral 37, and comprised principally of small uncoatedpellets 38 and large coated pellets 39, corresponding approximately tothe pellets 18 and 19 described in connection with the embodiment ofFIGURE l. However, the embodiment of FIGURE 2 employs additional ignitermaterial and features of construction not found in FIGURE l.

A first-stage ignition charge is in the form of a core 40, comprised ofrelatively rapid burning uncoated pellets 38, and housed in a tubularhousing 41, which is disposed coaXially with the cup 32, and has itslower end received in a central recess 42 in the inner wall of thebottom of the cup 32. Tubular container 41 may be either of metal or ofrelatively combustion resistant nonmetallic material, such as certainimpregnated papers, or certain plastics, It is necessary that the walls`of the negarse tubular container 41 delay combustion of that part of theignition charge 37 which surrounds the exterior of the container 41.

-.The first-stage ignition charge is supported within the container 41between transverse barriers 43 and 44,"which may be referred to asupstream and downstream barriers relative to the direction of ignitionburning. The barriers 43 and 44 may be of any suitable supportingconstruction, such as berboard, metal screen, foil discs or the like.The downstream barrier 44 is a cap which actually covers the inner endof tubular container 41, at a plane transverse to the igniter housing 31substantially ilush with the inner opening of the igniter housing cup32.

The second-stage ignition charge 45 is comprised of coated pellets 39and substantially lls the ignition housing cap 33 except for a thicklayer of plastic foam (such as Styrofoam) cushioning 46 between thesecond-stage ignition charge 45 and the interior wall surface of theinner end of the ignition housing cap 33. Both the second-stage ignitioncharge 45 and the cushioning layer 46 are enclosed in amoisture-impervious liner 47 and a disc shaped sheet 48 covering thecontents of the cap 33 and protecting them from exposure. It will beseen that the liner 47 extends into a ange 49 which serves as a gasketbetween the threaded cap 33` and the externally threaded cup 32.

The third-stage ignition charge 50 is comprised of a large number ofrelatively small uncoated rapidly burning pellets 51 embedded in a solidmatrix 52 of combustible material which may be the same as propellant 63and 64 or have burning properties intermediate between the propellantfuel and that of the ignition pellets 51. 1t will be seen that thematrix 52 is an annular body filling the lower part of the space betweenthe exterior of the tubular container 41 and the interior surface of thecup 32. The most convenient method of manufacture is to place thetubular container 41 in position, and till the annular space in cup 32to the desired depth with the matrix material in liquid form. Thecombustible pellets 51 are then inserted into the liquid matrixmaterial, which is then permitted to solidify into the thirdstageignition charge 50.

It is a preferred species of the invention to insure the proper ignitionof the third-stage ignition charge 50 by placement of the upper layer ofthe pellets 51 with exposed edges 53 projecting above the upper surfaceof the solid matrix 52.

An annular cushion 54 of plastic foam (such tas Styrofoam) covers theexposed edges 53 of the thirdstage ignition pellets 50 and cushions themfrom the intermediate stage coated pellets 39.

In operation, the igniter is actuated by the heating of the squib means36. The heat in `turn ash ignites squib powder charge 36h contained inthe recess 42 under the upstream barrier 43. The first-stage ignitioncharge 40 is rapidly ignited, and rapidly burns through the downstreambarrier 44 to ignition of the second-stage ignition charge 45.Combustion of the second-stage ignition charge 45 is prolonged becauseit is comprised of relatively large coated ignition pellets 39.

As ignition of the second-stage ignition charge 45 proceeds, thecushions 46 and 54 are rapidly consumed and ignition of the exposededges 53 of the third-stage ignition charge 50 occurs. Ilhe burning ofthe third ignition stage 50 is most prolonged of all the stages becauseof the envelopment of the pellets 51 in the solid matrix of relativelyslow burning material 52. The precise nature of its burning can bedetermined by the mixture of solid matrix 52 and quick burning pellets51.

It will be seen from the foregoing that I have provided an igniter whichfunctions as if it had three stages of ignition charge, by virtue of theconstruction of the igniter charge. The multi-stage eect is 'achieved byenveloping at least some of the pellets of the ignition charge inrelatively less combustible material. In the embodiment of FIGURE l, thesuppression of combustion in order to achieve a prolonged burning at alower and atter pressure peak is achieved by coating the pellets whichare not in the immediate vicinity of the starting squib 16. In theembodiment of FIGURE 2, the envelopment of the thirdstage pellets isaccomplished by embedding them in a relatively less combustible solidmatrix.

Although the envelopment of stages subsequent to the first or startingstage in relatively less combustible material is one essential part ofthe invention, itis to be understood that a second essential feature isthe structural distribution of the diilerent types of pellet materials.They are not placed at random, but are arranged with a first-stagereadily ignitible core enclosed in, or surrounded by, or at leastpartially surrounded by a second-stage or p several later stages ofenveloped material.

The preferred form makes use of the core and annulus` arrangement.

Furthermore, it is .a preferred species to employ the arrangement ofFIGURE 2 in which the solid annular matnix is actually separated fromthe first-stage ignition charge 40 by a metal combustion resistant tube41. However, it will be appreciated, that tube 41 might be constructedof material which had very little resistance, or might be dispensed withentirely if no intermediate-stage 45 were used; in that event, thecombustion would spread from the igniter core 40 directly to the solidmatrix 52, and the pellets 51 therein, in the same manner as occurs inthe embodiment of FIGURE 1.

Although I have described and illustrated two specific embodiments of myinvention, it will be understood that those skilled in the engineeringarts of ordnance and explosives will be able to conceive of manyvariations of the invention without departing from its spirit. Itherefore intend that the scope of the invention be dened not by thedetailed limitations set forth in the description of the embodiments ofFIGURES l and 2, but by the terms of the following claim.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is: A propellent igniter for projecting `astream of thot igniting gas onto propellant, which igniter includes: asubstantially cylindrical igniter housing having a nozzle end and asquib end, said housing having suflicient bursting strength to remainintact during burningl oef igniter materials lin its interior; wallsdefining a plurality of nozzle openings in said nozzle end for directingignition gases tow-ard the propellant to be ignited; a tubular initialstage charge container extending into the interior of said igniterhousing from the central part of said squib end, and having an openingfor discharge toward the interior of said nozzle end; said containerhaving an annular space between its exterior walls and the interior sidewalls of said igniter housing; an initial igniter charge of rapidlyburningigniter material located in said tubular container; squib meansmounted in the squib end of said igniter housing for ignition of saidinitial igniter charge; a middle stage charge of pellets slower-burningthan said initial charge, said middle stage charge being located in thenozzle end of s-a1d igniter housing and in the path of ignition gasesissuing from the discharge opening of said tubular container; a finalstage igniter charge in the form of an annular ring located in saidannular space between said tubular container yand the inner side wallsof said igniter housing, said nal stage igniter charge being comprisedof a large number of rapid burning igniter pellets embedded in a solidmatrix of material slow burning relative to said embedded pellets, withmany of said rapid burning igniter pellets having exposed portionsprojecting from the surface of said solid matrix toward said middlestage igniter charge; and a layer of cushioning material between saidmiddle stage charge and said nal stage igniter charge.

References Cited in the le of this patent 2,561,670

Y UNITED STATES PATENTS 2,072,671 Famke Mar. 2, 1937 2926'607 2,124,201Lewis July 19, 193s 5 2935948 2,398,683 Whitworth Apr. 16, 1946 MillerJuly 24, 1951 Sage Aug. 10, 1954 Gey Feb. 10, 1959 Muller Mar. 1, 1960Porter May 10, 1960

